Forest litter production, chemistry, and decomposition following two years of free-air CO₂ enrichment

Increases in tree biomass may be an important sink for CO₂ as the atmospheric concentration continues to increase. Tree growth in temperate forests is often limited by the availability of soil nutrients. To assess whether soil nutrient limitation will constrain forest productivity under high atmospheric CO₂, we studied the changes in forest litter production and nutrient cycling in a maturing southern U.S. loblolly pine-hardwood forest during two years of free-air CO₂ enrichment. The objective of this paper is to present data on the chemistry of green leaves and leaf litter, nutrient-retranslocation efficiency, aboveground litter production, whole-system nutrient-use efficiency, decomposition, and N availability in response to forest growth under elevated CO₂. The chemical composition of green leaves and leaf litter was largely unaffected by elevated CO₂. Green-leaf nitrogen (N) and phosphorus (P) concentrations were not significantly lower under elevated CO₂. N and P retranslocation from green leaves did not increase under elevated CO₂; therefore, leaf litter N and P concentrations were not significantly lower under elevated CO₂. The concentrations of carbon, lignin, and total nonstructural carbohydrates in litter were not significantly different under elevated CO₂. Total aboveground litterfall increased significantly with CO₂ fumigation. The increase in litterfall was due to significant increases in loblolly pine leaf litter and bark production. The mass of leaves from deciduous species did not increase with CO₂ fumigation. Whole-system nutrient-use efficiency (aboveground litterfall/nutrient content of litterfall) did not increase as a consequence of forest growth under elevated CO₂, but N and P fluxes from vegetation to the forest floor increased significantly. During the second year of CO₂ fumigation, the flux of N and P to the forest floor in litterfall increased by 20% and 34%, respectively. The rate of mass loss during one year of decomposition was unaffected by "litter type" (whether the litter was produced under ambient or elevated CO₂), nor by the "site" of decomposition (whether the litter was decomposed in the ambient or elevated CO₂ plots). N was immobilized in litter during decomposition, whereas P was mineralized. There was no consistent effect of litter type or site on nutrient dynamics in decomposing litter. There was no significant effect of elevated CO₂ on the pool size of inorganic N (NH₄⁺ and NO_3-^-) in the top 7.5 cm of mineral soil. The rate of net N mineralization and nitrification in mineral soil was not significantly different between treatment and control plots. Identifying the source of the nutrients lost in litterfall is critical to the long-term potential growth stimulation of forests under elevated CO₂. If the nutrients lost from biomass come from storage (e.g., the movement of nutrients from wood to leaves), then the increase in litter production should decrease over time as slowly replenished nutrient reserves are exhausted. If the nutrients lost in plant litter are replaced by uptake from soils, then it is possible (1) that trees acquire soil nutrients at a rate commensurate with growth stimulated by elevated CO₂; and (2) that forest productivity will be stimulated by elevated CO₂ in the long term.